[FFmpeg-user] Command line to keep original aspect ration and encode to iPhone with borders

[DM]

We need a command line to keep any original aspect ration and encode to
iPhone (.mp4), but with black borders.

We have customers uploading lots of different aspect ratio inputs, and only
the same one command line to enable the video
to play in any iPhone (and Android if possible)

So we can't have a command line with a changing border size, as we can't
change the command line for each upload.